Dissolved organic matter binding sequence with Cd trigger Brassica napus soil Cd spatial migration in rhizosphere

The binding sequences of dissolved organic matter (DOM) with cadmium (Cd) were important for Cd migration. However, it is still unknown which DOM components have the most significant binding sequence responses to the spatial distribution of Cd concentration in rhizosphere. In this study, the spatial distribution characteristics of binding sequences of soil DOM with Cd, as well as their coupling relationship with soil Cd concentration, were investigated for the first time. After 203 days of rhizobox cultivation, soil Cd concentration from Yiyang ranked as upper layers > lower layers, and rhizosphere zones > nonrhizosphere zones. The binding sequences of DOM with Cd, instead of DOM-Cd complexation amount, significantly drive the spatial migration of Cd. The binding sequences of high molecular weight (HMW) humus (r = 0.451*, p < 0.05), phenolic hydroxyl (r = 0.416*, p < 0.05) and amide groups (r = -0.423*, p < 0.05) with Cd(II) showed significant relationship with soil Cd concentration. The prevenient binding of HMW humus and phenolic hydroxyl groups with Cd could restrain the enrichment of Cd, wheras amide groups exhibited the opposite effect. Plate experiment profoundly proved that HMW and high hydroxyl richness DOM restrained soil Cd migration by coexisting with Cd. The mechanistic and quantitative insights into the effects of DOM conponents binding sequence with Cd on soil Cd dynamic will help reduce uncertainty in screening key activators.